Original ArticlesGrain Orientation Effect on Microstructure in Tensile Strained Copper
Introduction
Recently, the microstructural evolution in a tensile strained pure polycrystalline aluminum, with a grain size of 300 μm, was characterized by analyzing the microstructure of individual grains 1, 2. It was found that the observed microstructures could be grouped into three types with different characteristics [2]as follows.
Type 1: The grains are subdivided by long and straight dislocation boundaries on crystallographic {111} planes. These dislocation boundaries delineate cell blocks which contain ordinary dislocation cells.
Type 2: The grains are subdivided by ordinary dislocation cell boundaries defining a three-dimensional cell structure.
Type 3: The grains are subdivided as for Type 1, but the dislocation boundaries are not on crystallographic {111} planes.
It has also been seen that there is a clear correlation between the type of microstructure and the grain orientation, which was found to be, to a great extent, comparable with that obtained in tensile strained single crystals of copper where correlation was observed between the crystallographic orientation and the microstructural evolution 3, 4, 5, 6, 7, 8, 9, 10.
In tensile strained polycrystalline copper specimens, a grain to grain variation in the deformation microstructure has also been observed 11, 12. For example, equiaxed cells and parallel dislocation walls have been found in different grains of an oxygen-free high purity copper [11]. These variations have been related to the slip patterns predicted on the basis of a Schmid factor analysis. However, only a limited number of grains have been examined and the exact grain orientation data were not included in [11]. Thus, a clear relationship between the microstructure and grain orientation has not been established in polycrystalline copper, and this has been the aim of the present study.
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Experimental
The material used was oxygen free high conductivity (OFHC) copper (99.999% purity). The starting material was cold drawn to a rod with a diameter of 3.95 mm and then recrystallized to a grain size of about 50 μm. Optical microscopy revealed that twin boundaries exist in many grains [13]. The grain size measurement was carried out using the linear intercept method without taking the twin boundaries into account. Tensile specimens with 25 mm gauge length were cut from the rod. Tensile tests were
Results
In the present work, detailed and careful analysis of the deformation structure was conducted for 83 grains from the specimens deformed at the four strains used. The crystallographic orientations of the tensile axes of these grains are shown in an inverse pole figure of Fig. 1. Microstructural observations revealed that the deformation microstructure within a grain is basically homogeneous but significant grain to grain variations exist. The grain boundaries as well as the preexisting twin
Discussion
For the single crystals, very detailed studies of the deformation microstructures have been done in different sections of the deformed crystals. The orientations of the single crystals examined 3, 4, 5, 6, 7, 8, 9, 10are shown in Fig. 5. The crystals within the triangle are numbered to relate to the references, see Table 1. Steeds [3]and Humphreys and Martin [5]studied the microstructures for crystals 1 [3]and 2 and 3 [5], all having an orientation within the triangle. The TEM foils were
Conclusion
The deformation microstructures of 83 individual grains have been analyzed in tensile strained OFHD copper with grain size of about 50 μm.
(1) Three types of microstructures have been identified and a clear correlation has been found between the type of deformation microstructure and the grain orientation.
(2) The microstructure-orientation correction observed in the present experiment is in very good agreement with the behavior observed in coarse grained aluminum and in single crystals of copper
Acknowledgements
The author would like to thank Drs. N. Hansen, D. Juul Jensen, Q. Liu, W. Pantleon, G. Winther and A. Borrego for helpful discussions. J. Lindbo is gratefully acknowledged for skilled preparation of the TEM foils.
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